Is There Such a Thing as a Biosignature?

Reciprocating thermochemical mediator of pre-biotic polymer decomposition on mineral surfaces

A continuing frustration for origin of life scientists is that abiotic and, by extension, pre-biotic attempts to develop self-sustaining, evolving molecular systems tend to produce more dead-end substances than macromolecular products with the necessary potential for biostructure and function - the so-called 'tar problem'. Nevertheless primordial life somehow emerged despite that presumed handicap. A resolution of this problem is important in emergence-of-life science because it would provide valuable guidance in choosing subsequent paths of investigation, such as identifying pre-biotic patterns on Mars. To study the problem we set up a simple non-equilibrium flow dynamical model for the coupled temperature and mass dynamics of the decomposition of a polymeric carbohydrate adsorbed on a mineral surface, with incident stochastic thermal fluctuations. Results show that the model system behaves as a reciprocating thermochemical oscillator. The output fluctuation distribution is bimodal, with a right-weighted component that guarantees a bias towards detachment and desorption of monomeric species such as ribose, even while tar is formed concomitantly. This fluctuating thermochemical reciprocator may ensure that non-performing polymers can be fractionated into a refractory carbon reservoir and active monomers which may be reincorporated into better-performing polymers with less vulnerability towards adsorptive tarring.

Can Chirality Answer Whether We Are Alone?

Detecting biosignatures of life in extraterrestrial environments remains one of the primary objectives of scientific inquiry. Currently, both remote and direct detection methods are primarily aimed at identifying key molecular classes fundamental to terrestrial biology. However, a more universally applicable spectroscopic approach could involve searching for homochiral molecules. Thus, this perspective delves into the significance of homochirality as a critical factor in the origin of life. Without homochirality, the formation of self-recognizing and self-replicating complex molecules would be hindered. The various hypotheses concerning the origin of homochiral molecules have been explored and analyzed within this context. This perspective emphasizes the potential for discovering extraterrestrial microscopic life through the detection of homochiral molecules using chirodetecting methods such as chromatography and chiroptical spectroscopy or circular polarimetry as a promising remote technique. This discussion highlights the importance of homochirality in the broader search for life beyond Earth and underscores the need for innovative methodologies and instrumentation in astrobiological research. These techniques can be an effective method for detecting homochirality on future planetary missions.

Is Life Binary or Gradual?

The binary nature of life is deeply ingrained in daily experiences, evident in the stark distinctions between life and death and the living and the inert. While this binary perspective aligns with disciplines like medicine and much of biology, uncertainties emerge in fields such as microbiology, virology, synthetic biology, and systems chemistry, where intermediate entities challenge straightforward classification as living or non-living. This contribution explores the motivations behind both binary and non-binary conceptualizations of life. Despite the perceived necessity to unequivocally define life, especially in the context of origin of life research and astrobiology, mounting evidence indicates a gray area between what is intuitively clearly alive and what is distinctly not alive. This prompts consideration of a gradualist perspective, depicting life as a spectrum with varying degrees of "lifeness". Given the current state of science, the existence or not of a definite threshold remains open. Nevertheless, shifts in epistemic granularity and epistemic perspective influence the framing of the question, and scientific advancements narrow down possible answers: if a threshold exists, it can only be at a finer level than what is intuitively taken as living or non-living. This underscores the need for a more refined distinction between the inanimate and the living.

How to identify cell material in a single ice grain emitted from Enceladus or Europa

Icy moons like Enceladus, and perhaps Europa, emit material sourced from their subsurface oceans into space via plumes of ice grains and gas. Both moons are prime targets for astrobiology investigations. Cassini measurements revealed a large compositional diversity of emitted ice grains with only 1 to 4% of Enceladus's plume ice grains containing organic material in high concentrations. Here, we report experiments simulating mass spectra of ice grains containing one bacterial cell, or fractions thereof, as encountered by advanced instruments on board future space missions to Enceladus or Europa, such as the SUrface Dust Analyzer onboard NASA's upcoming Europa Clipper mission at flyby speeds of 4 to 6 kilometers per second. Mass spectral signals characteristic of the bacteria are shown to be clearly identifiable by future missions, even if an ice grain contains much less than one cell. Our results demonstrate the advantage of analyses of individual ice grains compared to a diluted bulk sample in a heterogeneous plume.

The Call for a New Definition of Biosignature

The term biosignature has become increasingly prevalent in astrobiology literature as our ability to search for life advances. Although this term has been useful to the community, its definition is not settled. Existing definitions conflict sharply over the balance of evidence needed to establish a biosignature, which leads to misunderstanding and confusion about what is being claimed when biosignatures are purportedly detected. To resolve this, we offer a new definition of a biosignature as any phenomenon for which biological processes are a known possible explanation and whose potential abiotic causes have been reasonably explored and ruled out. This definition is strong enough to do the work required of it in multiple contexts-from the search for life on Mars to exoplanet spectroscopy-where the quality and indeed quantity of obtainable evidence is markedly different. Moreover, it addresses the pernicious problem of unconceived abiotic mimics that is central to biosignature research. We show that the new definition yields intuitively satisfying judgments when applied to historical biosignature claims. We also reaffirm the importance of multidisciplinary work on abiotic mimics to narrow the gap between the detection of a biosignature and a confirmed discovery of life.